To an increasing extent, touch-sensitive panels are being used for providing input data to computers, electronic measurement and test equipment, gaming devices, etc.
In one category of touch-sensitive panels, denoted “integrated touch-sensing displays” in the following, an array of light-sensitive elements is integrated in a display screen or panel, whereby the location of an object touching the display screen is detected based on output signals from the individual light-sensitive elements. Integrated touch-sensing displays are generally designed to have a form factor similar to ordinary displays, and therefore the array of light-sensitive elements is arranged in proximity of the array of image-generating elements (also denoted “pixel-defining elements” or “picture elements” in the following) in the depth direction of the display (i.e. perpendicular to the front surface of the display). Generally, the spacing between the arrays of light-sensitive elements and picture elements in the depth direction is less than about 15 cm, and often the light-sensitive elements and the picture elements are interleaved on a common substrate or at least in a common geometric plane within the display.
WO2007/058924 discloses such an integrated touch-sensing LCD (Liquid Crystal Display), in which a liquid crystal material is arranged between a front electrode layer and a rear electrode layer. By changing the potential applied to pixel-defining thin-film resistors (TFTs) on the rear electrode layer, the liquid crystal material is modified locally such that light from a backlight behind the rear electrode is selectively transmitted to a front surface of the display. The rear electrode layer also includes an array of light-sensing TFTs, which are designed to detect ambient light impinging on the front surface of the display. A processor determines the position of an object touching the front surface, by identifying light-sensing TFTs that have been inhibited from sensing ambient light. It is also suggested to supplement this shadow detection by a detection of visible light diffused from a light guide arranged on top of the liquid crystal material. A light source is controlled to inject visible light in the light guide to propagate therein by total internal reflection. When the object touches the light guide, the internally reflected light will be diffused towards the light-sensitive elements. The position of the object may then be determined from a combination of transmitted ambient light, light diffused from the waveguide and light transmitted from the backlight and reflected on the object.
Another integrated touch-sensing LCD is known from US2008/0074401, in which infrared light-sensing TFTs are included among pixel-defining TFTs on a lower substrate. A backlight is provided behind the lower substrate to produce visible and infrared light. A transparent window is aligned with each light-sensing TFT. An object touching the display will locally reflect the infrared light from the backlight back through one or more windows onto the light-sensing TFTs. Touch points are determined based on the output signals of the light-sensing TFTs.
In US2007/0296688, photosensors are disposed in a liquid crystal panel, and a backlight is disposed on a back side of the panel. The panel is controlled to operate in different detection modes based on the amount of ambient light. When the illuminance of ambient light is high, touching objects are identified as shadows in the ambient light reaching the photosensors. When the illuminance of ambient light is low, the backlight is controlled to emit invisible light and touching objects are identified based on reflected light reaching the photosensors.
The integrated touch-sensing displays of the prior art suffer from at least one of a number of different limitations. Such limitations include sensitivity to variations in ambient light, difficulty to distinguish between non-touching and touching objects, insufficient signal-to-noise ratio (SNR), reduced image quality (e.g. contrast, brightness, spatial resolution) compared to an ordinary display (i.e. without touch-sensitivity), complex determination of positions for touching objects, complex device control, and need for usage restrictions.
Similar limitations may be equally relevant to integrated touch-sensing displays that are based on other display technologies, such as plasma displays and OLED (Organic Light Emitting Diode) displays.
The prior art also comprises US2008/0029691 which discloses a touch surface formed by a waveguide that receives light that undergoes total internal reflection. A separate camera is arranged behind the waveguide to capture images of the waveguide surface. Touches on the front surface of the waveguide are detected by the camera detecting light that escapes the waveguide due to frustration of the total internal reflection. This touch-sensing apparatus has quite a different form factor than the above-described integrated displays, since the camera needs to be arranged with significant spacing from the waveguide in order to capture the images of the waveguide surface.